Kaolin pigments, their preparation and use

ABSTRACT

A pigment product suitable for use in a coating composition to provide a gloss coating on paper, the pigment product comprising a processed particulate kaolin produced from a naturally platy kaolin clay having a particle size distribution such that about 50% to 60% by weight of the particles have an esd less than 2 μm and less than 25% by weight of the particles have an esd less than 0.25 μm and a shape factor of at least 15. This platy clay can be blended with b-fraction clay component. In processing, the crude kaolin is ground to a shape factor greater than 50 to produce even greater “platy” clay. The pigment product has a particle size distribution such that at least 80% by weight of the particles have an esd less than 2 μm and greater than 12% and less than 35% by weight of the particles have an esd less than 0.25 μm and whose surface area is greater than 12 m 2 /g when measured by the BET method.

This application is a national phase application based on internationalapplication number PCT/US00/08466, filed Mar. 31, 2000, and claims thebenefit of provisional application No. 60/127,380, filed Apr. 1, 1999,the contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to pigment products, and their productionand use in coating compositions for preparing gloss coated paper,especially lightweight and ultra-lightweight coated paper. Moreparticularly, the invention relates to paper coating pigments made froma crude feed comprising a naturally platy clay having a particle sizedistribution such that greater than 50% by weight of the particles havean esd less than 2 microns and less than 25% by weight of the particleshave an esd less than 0.25 microns and a shape factor of 15 or more.

2. Background of the Invention

Paper coating compositions are generally prepared by forming a fluidaqueous suspension of pigment material together with a hydrophilicadhesive and other optional ingredients.

Lightweight coated, or LWC, paper is generally coated to a weight offrom about 5 g·m⁻² to about 13 g·m⁻² on each side, and the totalgrammage, or weight per unit area of the coated paper is generally inthe range of from about 49 g·m⁻² to about 65 g·m⁻². The coating mayconveniently be applied by means of a coating machine including a shortdwell time coating head, which is a device in which a captive pond ofcoating composition under a slightly elevated pressure is held incontact with a moving paper web for a time in the range of from 0.0004second to 0.01 second, before excess coating composition is removed bymeans of a trailing blade. However, other types of coating apparatus mayalso be used for preparing lightweight coated paper. LWC paper isgenerally used for printing magazines, catalogues and used for preparinglightweight coated paper. LWC paper is generally used for printingmagazines, catalogues and advertising or promotional material. Thecoated paper is required to meet certain standards of surface gloss andsmoothness. For example, the paper is generally required to have a glossvalue of at least about 32, and up to about 60, TAPPI units, and aParker Print Surf value in the range of from about 0.5 to about 1.6 μm.

Ultra lightweight coated, or ULWC, paper is sometimes otherwise known aslight lightweight coated, or LLWC, paper and is used for catalogues andfor advertising and promotional material sent through the mail to reducemailing costs. The coating weight is generally in the range of from 5g·m⁻² to 7 g·m⁻² per side. The grammage is generally in the range offrom about 35 g·m⁻² to about 48 g·m⁻².

Rotogravure printing generally involves the use of an engraved or etchedcylinder as an image carrier. Image areas are etched or engraved belownon-image areas in the form of tiny sunken cells. The cylinder isimmersed in ink, and the excess ink is scraped off by a blade. When thesubstrate contacts the printing cylinder, ink transfers, forming theimage.

Offset printing is an indirect printing method in which the inked imageon a press plate is first transferred to a rubber blanket that, in turn,“offsets” the inked impression to a press sheet.

A very important white inorganic pigment for use in preparing coatingcompositions for the manufacture of LWC and ULWC papers for rotogravureor offset printing is kaolin obtained from kaolin clay. Large depositsof kaolin clay exist in Devon and Cornwall, England and in the States ofGeorgia and South Carolina, United States of America. Important depositsalso occur in Brazil, Australia, and in several other countries.

Kaolin clay, also referred to as china clay or hydrous kaolin, consistspredominantly of mineral kaolinite (Al₂Si₂O₅(OH)₄), an hydrous aluminumsilicate, together with small proportions of various impurities.

Some of these impurities, such as fine ferruginous or titaniferousimpurities impart undesirable color to the clay. Other impurities, suchas mica, smectite, vermiculite, hydrobiotite, mixed or layeredillite-smectite or mixed layers of clay minerals generally have anundesirable effect on the rheology of the kaolin clay-water viscosity.In order to eliminate or to reduce these impurities, the kaolin crude issubject to one or several beneficiation steps, most of which are wellknown to the mineral processing industry.

Kaolinite exists in the form of hydrous aluminosilicate crystals in theshape of thin hexagonal plates or booklets of platelets called “stacks”.The individual plates may have mean diameters of 1 μm or less, butkaolinite particles in the form of stacks of plates may have anequivalent spherical diameter (“esd”) of 10 μm or more. Generallyspeaking, kaolin clay particles which have an esd of 2 μm or more are inthe form of stacks of kaolinite plates, rather than individual plates.

As long ago as 1939, Maloney disclosed in U.S. Pat. No. 2,158,987 thatthe finish, or gloss, of a clay coated paper is greatly improved if theclay, before incorporation in the coating composition, is treated sothat a large percentage, for example 80% by weight or more, of the clayparticles have a size in the range of 0.1 μm to 2 μm. In order toincrease the proportion of fine particles in the raw kaolin, the rawkaolin may, according to the disclosure in U.S. Pat. No. 2,158,987 besubjected, before the centrifuging step, to a grinding or delaminatingoperation in which a suspension containing from about 50% to about 75%by dry weight of kaolin and a dispersing agent is subjected to pebblemilling. When the kaolin from the finer fraction is recovered, mixedwith a suitable paper coating binder, and applied to the surface of abase paper, a coating of good gloss and color is obtained.

Various pigment products which are made using the principles describedby Maloney in U.S. Pat. No. 2,158,987 are commercially available andprovide good gloss and smoothness in coated papers, especially for LWCand ULWC paper. For example, a pigment product available from ImerysMinerals Ltd, formerly ECC International Ltd., a British corporation,and recommended for gloss coatings of LWC consists of a refined Englishkaolin product having a particle size distribution, “psd”, such that 89%by weight of the particles have an esd less than 2 μm, 74% by weight ofthe particles have an esd less than 1 μm and 25% by weight of theparticles have an esd less than 0.25

A kaolin product of high shape factor is considered to be more “platy”than a kaolin product of low shape factor. “Shape factor” as used hereinis a measure of an average aspect ratio value (on a weight averagebasis) of the ratio of mean particle diameter to particle thickness fora population of particles of varying size and shape as measured usingthe electrical conductivity method and apparatus described inGB-A-2,240,398/U.S. Pat. No. 5,1286,06/EP-A-0,528,078 and in U.S. Pat.No. 5,516,617 and using the equations derived in these patentspecifications. “Mean particle diameter” is defined as the diameter of acircle which has the same area as the largest face of the particle. Inthe measurement method described in GB-A-2240398/U.S. Pat. No.5,128,606/EP-A-0528078, the electrical conductivity of a fully dispersedaqueous suspension of the particles under test is caused to flow throughan elongated tube. Measurements of the electrical conductivity are takenbetween (a) a pair of electrodes separated from one another along thelongitudinal axis of the tube, and (b) a pair of electrodes separatedfrom one another across the transverse width of the tube, and using thedifference between the two conductivity measurements the shape factor ofthe particulate material under test is determined.

The kaolin deposits in England are of primary kaolin, whilst those inthe USA are of both the primary and the sedimentary (secondary) types.Kaolin was formed in geological times by the hydrothermal decompositionor by the weathering of the feldspar and mica components of granite andfeldspathic metamorphic rocks, and primary kaolin is that which isobtained directly from the granite matrix in which it was originallyformed. On the other hand, secondary kaolin, also known as sedimentarykaolin, has been washed out of the original granite matrix in geologicaltimes and has been deposited in an area remote from the site in which itwas originally formed. Secondary kaolin deposits tend to have a higherproportion of fine particles, with an esd smaller than about 2 μm,because the kaolin has undergone a certain amount of natural grindingand sorting during the course of its transport from its site of originto its site of final deposition. Jepson (Jepson, W B, “Kaolins: theirproperties and uses”, Phil. Trans. R. Soc. Lond., A311, 1984, pp411-432) has shown that a sample of an English primary kaolin clay,which has been subjected to a particle size separation such thatsubstantially all particles having an esd larger than 5 μm have beenremoved, will comprise particles in the form of roughly hexagonal plateshaving diameters in the range of from about 8 μm down to about 0.1 μm,with shape factors which will vary from the coarsest particles to thefinest particles the average shape factor being from about 20 to about30. On the other hand, a sample of secondary kaolin clay from Georgia,USA, which has been subjected to a particle size separation such thatsubstantially all particles having an esd larger than 5 μm have beenremoved, will typically comprise particles which more nearly conform toa regular hexagonal shape and which had diameters of 0.1 μm and above,but a generally finer distribution of diameters than is the case withthe English kaolin. The shape factor of the particles of the Georgia,USA kaolin will lie within the range of from about 6 to about 10, butthere will be little variation in the shape factor with particlediameter.

The aforesaid kaolin products, as well as commercially available kaolinproducts, generally are produced from a kaolin crude which is mined fromthe secondary clay deposit beds which have been selected to have goodrheological characteristics. In the past, the kaolin claymineral-producing industries have avoided using other clay beds in thecrude ores because they were considered inferior due to rheolologyproblems. The rheology problems are considered to be associated withplatiness and higher levels of impurities. In general, the crude ore inthe normally rejected zones of the clay beds is a naturally platy clayhaving a shape factor of greater than 15 which when processed, has ahigher viscosity and therefore “poor” viscosity compared to the crudeore taken from normally selected zones.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a kaolin clay pigment made froma naturally platy crude clay which generally is obtained from thenormally rejected zones of a sedimentary kaolin bed, such as those inWashington County, Georgia, U.S.A. The crude clay generally will have ashape factor of at least 15. The particle size distribution of thiscrude clay is such that greater than about 50% by weight, and in oneembodiment even greater than about 60% by weight, of the particles havean esd less than 2 microns and not more than 25% by weight of theparticles have an esd less than 0.25 microns.

According to a second aspect of the invention, there is provided amethod for producing a pigment according to the first aspect. Thismethod comprises the steps of:

(a) mixing a raw kaolin clay crude, comprising a naturally platy clayfrom a sedimentary deposit and having a shape factor of at least 15 andhaving a particle size distribution such that greater than 50% by weightof the particles have an esd less than 2.0 microns and less than 25% byweight of the particles have an esd less than 0.25 microns, with waterto form an aqueous suspension containing about 20% to about 70% solidsof kaolin on a dry weight basis;

(b) diluting to 40% solids of kaolin on a dry weight basis and thensubjecting the suspension to attrition grinding using a particulategrinding medium for a time sufficient to dissipate in the suspension anoptimum amount of energy to impart an average shape factor to the kaolinclay particles of at least 50;

(c) separating the suspension of ground kaolin clay from the particulategrinding. medium;

(d) subjecting the suspension of step (c) to a particle sizeclassification step; and

(e) dewatering the suspension of ground kaolin clay classified in step(d) to recover a pigment product therefrom.

In step (a) the formed aqueous suspension preferably contains about 60%solids of kaolin on a dry weight basis.

In step (e) and as a result of step (d), the kaolin clay particles mayhave a particle size distribution such that at least 80%, preferablybetween 85% to 95% by weight, and most preferably, 88% to 92% by weight,of the particles have an esd less than 2 microns and less than 35%preferably, from about 20% to about 35%, and most preferably, about 25%to about 35%, by weight of the particles have an esd less than 0.25microns.

In step (b), the energy dissipated in the suspension may range fromabout 20 kWh to about 100 KWh of energy per ton of kaolin present on adry weight basis.

Beneficiation steps to improve clay brightness, such as magneticseparation, ozone, reduced-acid leaching, flotation, or selectiveflocculation, may be performed on the clay suspension or slurry prior toor after the attrition grinding in step (b).

The pigment product of the invention preferably has a surface areagreater than 12 m²/g, more preferably between 15 m²/g to 20 m²/g whenmeasured by the BET method.

Since the pigment products of the invention are typically 80%-95% byweight finer than 2 micrometers, and the surface area may be greaterthan 15 m²/g when measured by the BET method, they can be classified asNo. 1 kaolin clay pigments. In step (a) the shape factor of thenaturally platy clay prior to any delamination or grinding thereof is atleast 15 and, preferably, is greater than 25, and most preferably isgreater than 30. As a result of the shape factor produced in step (b)being at least 50, the kaolin clay particles finer than 2 micrometersare extremely platy.

A second embodiment of the invention provides for a blend of kaolinclays to produce a pigment product. For example, the blend wouldcomprise about 80% of a naturally platy kaolin crude as described hereinhaving a shape factor of at least 15 and preferably greater than 25, andabout 20% of a “blocky” kaolin clay having a shape factor less than 10.This “blocky” clay may be a b-fraction kaolin clay component, whichgenerally is the discarded fraction in a centrifuge or classificationoperation in that it is considered to be “too coarse” for paper coatingapplications. This blend would be subjected to step (a) through step (e)in accordance with the method of the invention and the second aspectthereof.

The pigment product of the two embodiments of the invention has shown togive greater improvement in sheet brightness, opacity, and gloss ascompared to standard products in starch containing binder systems.Pigments of the invention are particularly useful for use incompositions for coating lightweight (LWC) and ultra-lightweight (ULWC)printed-paper.

DETAILED DESCRIPTION OF THE INVENTION

The novel hydrous kaolin pigments of the invention are adapted forcoating LWC and ULWC paper adapted to be printed by offset and rotogravure and comprise a major amount of platy kaolin particles where theplatiness of the kaolin particles is defined in terms of a “high” shapefactor, that is, the average shape factor of the particles of thepigment is at least 50, preferably 60.

The pigment product according to a first aspect of the present inventionmay be obtained by treating a raw particulate hydrous naturally platykaolin mineral obtained from the normally rejected (poor viscosity)zones of the sedimentary or secondary type of kaolin, such as that whichoccurs in Georgia, USA.

In the Georgia deposits, it is not common practice to utilize thenaturally platy clays because the rheology (viscosity) is poor comparedto the blocky clays generally available in the deposits. When a“naturally platy” clay is referred to in the invention, it is meant thatthe clay, generally having a shape factor of at least 15, will naturallygrind to a very high shape factor, generally greater than about 50, withrelatively low amounts of energy, from about 20 to about 100 KWh/ton ofclay, more about which is discussed hereinbelow.

The novel kaolin pigments resulting from step (e) of the methodaccording to the first aspect of the invention may have the followingcharacteristics:

Particle Size Distribution

At least 80% by weight, preferably 85% to 95% by weight, and mostpreferably 88% to 92% by weight, finer than 2 micrometers; and

58% by weight, preferably 50% to 60% by weight, and, most preferably 53%to 58% by weight, finer than 0.5 micrometers;

38% by weight, preferably 30% to 40% by weight, finer than 0.3micrometers;

less than 35% by weight, most preferably, 25% to 30% by weight, finerthan 0.25 micrometers.

Average particle size (such as d₅₀) ranging from about 0.30 to about0.60 micrometers, and, preferably, from about 0.35 to about 0.52, andmost preferably, 0.38 to about 0.45 micrometers.

Surface Area

BET surface area is preferably greater than 12 m²/g, more preferably isgreater than 15 m²/g, and even more preferably is greater than 16 m²/g.

Solids Content

The clay-water slurry weight percent solids of the pigment product ofthe invention may be at least 50% by weight, preferably ranging from61.0% to about 64.0% by weight of kaolin clay particles in dry weightform. The clay content of a coating composition comprising the pigmentproduct of the invention may range from about 65% by weight to about 90%by weight of the total dry solids.

Surface area is a property related to particle size of kaolins althoughsurface area alone does not correlate directly with particle size.Surface area is expressed in terms of square meters of area per gram ofa material and is frequently measured by the BET method using nitrogenas adsorbate.

The starting crude kaolin useful in the invention is naturally platykaolin. Such crudes contain particles having a shape factor equal to orgreater than 15, preferably, equal to or greater than 25. With a shapefactor less than 15, the crude is composed substantially of booklets. Ashape factor above 30 indicates that the clay is composed substantiallyof thin platelets.

The pigment product preferably has a particle size distribution “psd”such that at least 80%, preferably 85% to 95% by weight, and mostpreferably 88% to 92% by weight of the particles have an esd smallerthan 2 μm and less than 35% by weight of the particles have an esdsmaller than 0.25 μm. Desirably, at least 70%, preferably at least 72%by weight have an esd less than 1 μm. As will be appreciated by thoseskilled in the art, the psd of a particulate product such as the pigmentproduct according to the present invention may be determined bymeasuring the speeds at which dispersed particles of the particulateproduct under test, sediment through a standard dilute aqueoussuspension using a SEDIGRAPH™ instrument, for example the SEDIGRAPH5100, obtained from Micromeritics Corporation, USA. The size of a givenparticle is expressed in terms of the diameter of a sphere of equivalentdiameter, which sediments through the suspension is expressed as the esdor esd, the parameter as referred to above. The SEDIGRAPH instrumentsmeasures and graphically records the percentage by weight of particleshaving esd less than a certain esd value versus esd.

According to the second aspect of the invention, the method providesgrinding the particles of the naturally platy kaolin crude ore to ashape factor greater than 50. These natural platy kaolin clays in theinvention generally will contain about 50-60% by weight of particlesfiner than 2 μm and less than 25% by weight of particles finer than 0.25μm. The fine particles in these crudes, for example, the particles finerthan 2 μm esd contain very fine booklets or fine individual particles.In the invention, attrition grinding is applied to the crude particlesprior to a classification step in order to increase the number of fineindividual platelets. That is, the shape factor is increased from about15 to 25 to at least 50.

The method of the invention comprises the steps of:

(a) mixing a raw kaolin crude comprising a naturally platy clay having ashape factor of at least 15 and a particle size distribution such thatgreater than 50% by weight of the particles have an esd less than 2microns and not more than 25% by weight of the particles have an esdless than 0.25 μm, with water to form an aqueous suspension containingabout 60% solids of kaolin on a dry weight basis;

(b) diluting to 40% solids of kaolin on a dry weight basis andsubjecting the suspension to attrition grinding using a particulategrinding medium for a time sufficient to dissipate in the suspension anoptimum amount of energy to impart an average shape factor value to thekaolin clay particles of at least 50;

(c) separating the suspension of ground kaolin clay from the particulategrinding medium; and

(d) subjecting the suspension of step (c) to a classification step;

(e) dewatering the suspension to a ground kaolin clay separation in step(d) to recover a pigment product therefrom.

Between steps (a) and (b), the clay slurry is degritted to remove coarseparticles greater than 45 microns and the clay slurry is subjected to ahigh intensity magnetic separator to remove the iron-bearing mineralsfrom the clay.

Typically, in step (a) the crude is initially crushed and thenmechanically worked, for example, by blunging in water, preferablycontaining clay dispersant, such as one or more inorganic or organicagents well known in the art. In the invention, it is preferred to use amixture of sodium hexametaphosphate and soda ash. Generally, the solidsof the blunged clay are in the range of 20% to 65%, preferably fromabout 40% to 60% by weight of kaolin on a dry weight basis. The blungedclay slurry may be degritted by passing the slurry through sand boxesand a +325 mesh (U.S. standard) screen to remove to coarse (grit)particles larger than 45 microns. The slurry is then subjected to a highintensity magnetic separator to remove the iron-bearing minerals fromthe clay.

From the magnetic separator, and as described in step (b), the clayslurry is subjected to an attrition grinding step using a particulategrinding medium for a time sufficient to dissipate in the suspensionenough energy to impart an average shape factor value to the particlesof at least 50, which makes the naturally play clay crude even“platier”.

The particulate grinding medium preferably has a specific gravity of 2or more, and advantageously comprises grains of silica sand or similarmedia, the grains generally having diameters not larger than about 2 mmand not smaller than about 0.25 mm. Preferably, the amount of energydissipated in the suspension of kaolin clay is in the range of fromabout 20 kWh to about 100 KWh of energy per ton of kaolin present on adry weight basis. Typically, the amount of energy dissipated for theplaty crudes used in the invention will range from about 25 kWh to about50 kWh per ton of kaolin present on a dry weight basis.

After attrition grinding and as recited in step (b), the clay slurry maybe passed through a particle size separator such as a conventionalcentrifuge to classify the clay to a particle size distribution.

The 2 micron content after attrition or sand grinding generally variesfrom batch to batch of kaolin crude, and therefore, a classificationstep is employed to: 1) ensure that the pigment product has consistentparticle size; and 2) adjust the 2 micron content to a level needed fora desired performance of the pigment product.

In step (e), the suspension of ground kaolin may be dewatered in one ofthe ways well known in the art, such as filtration, centrifugation,evaporation and the like. Dewatering using a rotary vacuum filter ispreferred. For example, use of a rotary vacuum filter may be made toform a cake (product slurry) having a water range of from about 35% toabout 60% by weight. This cake may be mixed with a dispersing agent forthe kaolin clay and thus converted into fluid slurry. This slurry of thekaolin clay may be dried. In one embodiment of the invention, the kaolinclay is thermally dried by introducing the fluid slurry of the kaolinclay into a spray drier and thereby transported in a substantially dryform.

Alternatively, the solids concentration may be raised by evaporation orspray dry back mixing a portion of the fluid slurry to raise the solids.

Between steps (d) and (e), the suspension may be subjected to a leachingprocess to remove iron oxides and other leachable colorant species tochange the shade of the clay.

After the dewatering step (e), the solids content of the slurry may beraised to about 60%, or it may be lower or higher than 60%, by weight.This solids content represents “good” rheology for this kaolin product.

A further embodiment of the invention provides for a blend of kaolinclays. For example, the blend would comprise about 80% of a naturallyplaty kaolin crude as described hereinabove having a shape factor of atleast 15 and preferably greater than 25, and about 20% of a “blocky”kaolin clay having a shape factor less than 10. This “blocky” clay maybe a b-fraction kaolin clay component, which generally is discarded in acentrifuge or classification operation in that it is considered to be“too coarse” for paper coating applications. This blend would besubjected to step (a) through step (e) in accordance with the method ofthe invention and the second aspect thereof.

A second embodiment of the invention pertains to a blend of a naturallyplatey clay with a b-fraction kaolin clay component. This blend is suchthat the pigment has the desired particle size distribution as specifiedhereinabove. For example, the blend may comprise from 50% by weight to95% by weight, or preferably from 70% by weight to 90% by weight, of acrude platy clay as described herein. In addition, the blend maycomprise from about 5% to about 50%, or preferably from about 10% toabout 30% by weight of a coarse clay and may have less than 20% byweight less than 0.25 microns. The coarse clay may have greater than 60%by weight, especially greater than 70% by weight greater than 2 micronsand may have less than 20%, by weight less than 0.25 microns. The coarseclay may be obtained as the coarse fraction obtained by a particle sizeseparation.

The b-fraction clay component or coarse fraction from a centrifugeoperation may be blended with a naturally platy clay and ground to ahigh shape factor, generally greater than 50. This is helpful to controlthe <0.25 micron content of the product pigment to the desired level.

According to the present invention in a third aspect there is provided acoating composition for use in producing gloss coatings on paper andother substrates which composition comprises an aqueous suspension of aparticulate pigment together with a hydrophilic adhesive or binder,wherein the particulate pigment comprises the pigment product accordingto the first aspect of the invention.

The pigment product according to the two embodiments of the inventionmay be used in paper coating as follows.

The clay content of the paper coating composition according to the thirdaspect of the invention may be greater than 60% by weight, preferably atleast 70% of total dry solids, preferably as high as possible but stillgiving a suitably fluid composition which may be used in coating. Thecomposition may include a dispersing agent, for example up to 2% byweight of a polyelectrolyte based on the dry weight of pigment present.For example, polyacrylates and copolymers containing polyacrylate unitsare well known as suitable polyelectrolytes.

The pigment product (such as a naturally platy clay or a blend ofnaturally platy clay and a b-fraction), according to the two embodimentsof the first aspect of the invention may be used as the sole pigment inthe paper coating composition according to the third aspect, or it maybe used in conjunction with one or more other known pigments, such asfor example, (commercially available) kaolin, calcined kaolin, naturalor precipitated calcium carbonate, titanium dioxide, calcium sulphate,satin white, talc and so called ‘plastic pigment’. When a mixture ofpigments is used, the pigment product (whether being 100% naturallyplaty clay or a blend of a naturally platy clay with b-fraction kaolinclay component), according to the two embodiments of the first aspect ofthe invention is preferably present in the mixture in an amount of atleast 80% of the total dry weight of the mixed pigments; however, theskilled artisan can readily modify these amounts under appropriatecircumstances to achieve the desired pigment properties.

The binder of the composition according to the third aspect mayconveniently comprise an adhesive derived from natural starch obtainedfrom a known plant source, for example, wheat, maize, potato or tapiocaalthough it is not essential to use starch as a binder ingredient. Otherbinders, which may be used with or without starch, are mentioned later.

Coating application for paper which will be printed by the rotogravuremethod described earlier, usually employs synthetic binders in thecoating composition. Where the coated paper will be printed by theoffset method, natural binders described above, like starch, arecommonly used. Where starch is employed as a binder ingredient, thestarch may be unmodified or raw starch, or it may be modified by one ormore chemical treatments known in the art. The starch may, for example,be oxidized to convert some of its —CH₂OH groups to —COOH groups. Insome cases the starch may have a small proportion of acetyl, —COCH₃,groups. Alternatively, the starch may be chemically treated to render itcationic or amphoteric (having both cationic and anionic charges) Thestarch may also be converted to starch ether, or hydroxyalkylated starchby replacing some —OH groups with, for example, —O·CH₂O·CH₂OH groups,—O·CH₂O·CH₃ groups or —O·CH₂O·CH₂O·CH₂OH groups. A further class ofchemically treated starches, which may be used, is that known as thestarch phosphates. Alternatively, the raw starch may be hydrolyzed bymeans of a dilute acid or an enzyme to produce a gum of the dextrintype. The amount of the starch binder used in the composition accordingto the third aspect is preferably from 4% to 25% by weight, based on thedry weight of pigment. The starch binder may be used in conjunction withone or more other binders, for example synthetic binders of the latex orpolyvinyl acetate or polyvinyl alcohol type. When the starch binder isused in conjunction with another binder, such as a synthetic binder, theamount of the starch binder is preferably from 2% to 20% by weight, andthe amount of the synthetic binder from 2% to 12% by weight, both basedon the weight of dry pigment. Preferably, at least 50% by weight of thebinder mixture comprises modified or unmodified starch.

According to the present invention in a fourth aspect there is provideda method of use of the coating composition according to the third aspectwhich comprises applying the composition to coat a sheet of paper andcalendering the paper to form a gloss coating thereon. Preferably, thegloss coating is formed on both sides of the paper.

Calendering is a well known process in which paper smoothness and glossis improved and bulk is reduced by passing a coated paper sheet betweencalender nips or rollers one or more times. Usually, elastomer coatedrolls are employed to give pressing of high solids compositions. Anelevated temperature may be applied. One or more passes through the nipsmay be applied.

The paper after coating and calendering in the method according to thefourth aspect may have a total weight per unit area in the range 30g·m⁻² to 70 g·m⁻², especially 49 g·m⁻² to 65 g·m⁻² or 35 g·m⁻² to 48g·m⁻². The final coating preferably has a weight per unit areapreferably from 3 g·m⁻² to 20 g·m⁻², especially from 5 g·m⁻² to 13g·m⁻². Such a coating may be applied to both sides of the paper. Thus,the coated paper may be LWC or ULWC paper. The paper gloss may begreater than 45 TAPPI units and the Parker Print Surf value at apressure of 1mPa of each paper coating may be less than 1 μm.

The gloss of a coated paper surface may be measured by means of a testlaid down in TAPPI Standard No 480 ts-65. The intensity of lightreflected at an angle from the surface of the paper is measured andcompared with a standard of known gloss value. The beams of incident andreflected light are both at an angle of 75° to the normal to the papersurface. The results are expressed in TAPPI gloss units.

The Parker Print Surf test provides a measure of the smoothness of apaper surface, and comprises measuring the rate at which air underpressure leaks from a sample of the coated paper which is clamped, undera known standard force, between an upper plate which incorporates anoutlet for the compressed air and a lower plate, the upper surface ofwhich is covered with a sheet of either a soft or a hard referencesupporting material according to the nature of the paper under test.From the rate of escape of the air, a root mean cube gap in μm betweenthe paper surface and the reference material is calculated. A smallervalue of this gap represents a higher degree of smoothness of thesurface of the paper under test.

The pigment product of the two embodiments of the first aspect of theinvention has been shown to give greater improvement in sheetbrightness, opacity, print gloss, and gloss as compared to standardproducts in starch containing binder systems. However, an improvementmay also be obtained where other known starch-free binders are employed(with or without starch present). In each case the adhesive or bindermay form from 4% to 30%, and preferably from 8% to 20%, and even morepreferably from 8% to 15% by weight of the solids content of thecomposition. The amount employed will depend upon the composition andthe type of adhesive, which may itself incorporate one or moreingredients. For example, hydrophilic adhesives used in the art thatincorporate one or more of the following adhesive or binder ingredientsmay be used in the following stated amounts:

(a) latex: levels range from 4% by weight to 20% by weight. The latexmay comprise for example a styrene butadiene, acrylic latex, vinylacetate latex, or styrene acrylic copolymers.

(b) other binders: levels again range from about 4% by weight to about20% by weight. Examples of other binders include casein, polyvinylalcohol and polyvinyl acetate.

Additives in various known classes may, depending upon the type ofcoating and material to be coated, be included in the coatingcomposition according to the third aspect of the present invention.Examples of such classes of optional additive are as follows:

(a) cross linkers: generally in levels of up to 5% by weight; forexample glyoxals, melamine formaldehyde resins, ammonium zirconiumcarbonates.

(b) water retention aids: generally up to 2% by weight, for examplesodium carboxymethyl cellulose, hydroxyethyl cellulose, PVA (polyvinylacetate), starches, proteins, polyacrylates, gums, alginates,polyacrylamide bentonite and other commercially available products soldfor such applications.

(c) viscosity modifiers or thickeners: generally in levels up to 2% byweight; for example polyacrylates, emulsion copolymers, dicyanamide,triols, polyoxyethylene ether, urea, sulphated castor oil, polyvinylpyrrolidone, montmorillonite, CMC (carboxymethyl celluloses), sodiumalginate, xanthan gum, sodium silicate, acrylic acid copolymers, HMC(hydroxymethyl celluloses), HEC (hydroxyethyl celluloses) and others.

(d) lubricity/calendering aids: generally in levels up to 2% by weight,for example calcium stearate, ammonium stearate, zinc stearate, waxemulsions, waxes, alkyl ketene dimer, glycols.

(e) dispersants: generally in levels up to 2 per cent by weight, forexample polyelectrolytes such as polyacrylates and copolymers containingpolyacrylate species, especially polyacrylate salts (such as sodium andaluminum optionally with a group II metal salt), sodiumhexametaphosphates, non-ionic polyol, polyphosphoric acid, condensedsodium phosphate, non-ionic surfactants, alkanolamine and other reagentscommonly used for this function.

(f) antifoamers/defoamers: generally in levels up to 1% by weight, forexample blends of surfactants, tributyl phosphate, fatty polyoxyethyleneesters plus fatty alcohols, fatty acid soaps, silicone emulsions andother silicone containing compositions, waxes and inorganic particulatesin mineral oil, blends of emulsified hydrocarbons and other compoundssold commercially to carry out this function.

(g) dry or wet pick improvement additives: generally in levels up to 2%by weight, for example melamine resin, polyethylene emulsions, ureaformaldehyde, melamine formaldehyde, polyamide, calcium stearate,styrene maleic anhydride and others.

(h) dry or wet rub improvement and abrasion resistance additives:generally in levels up to 2% by weight, for example glyoxal basedresins, oxidized polyethylenes, melamine resins, urea formaldehyde,melamine formaldehyde, polyethylene wax, calcium stearate and others.

(i) gloss-ink hold-out additives: generally in levels up to 2% byweight, for example oxidized polyethylenes, polyethylene emulsions,waxes, casein, guar gum, CMC, HMC, calcium stearate, ammonium stearate,sodium alginate and others.

(j) optical brightening agents (OBA) and fluorescent whitening agents(FWA): generally in levels up to 1% by weight, for example stilbenederivatives.

(k) dyes: generally in levels up to 0.5% by weight.

(l) biocides/spoilage control agents: generally in levels up to 1% byweight, for example metaborate, sodium dodecylbenene sulphonate,thiocyanate, organosulphur, sodium benzonate and other compounds soldcommercially for this function such as the range of biocide polymerssold by Calgon Corporation.

(m) levelling and evening aids: generally in levels up to 2% by weight,for example non-ionic polyol, polyethylene emulsions, fatty acid, estersand alcohol derivatives, alcohol/ethylene oxide, sodium CMC, HEC,alginates, calcium stearate and other compounds sold commercially forthis function.

(n) grease and oil resistance additives: generally in levels up to 2% byweight, such as oxidized polyethylenes, latex, SMA (styrene maleicanhydride), polyamide, waxes, alginate, protein, CMC, HMC.

(o) water resistance additives: generally in levels up to 2% by weight,such as oxidized polyethylenes, ketone resin, anionic latex,polyurethane, SMA, glyoxal, melamine resin, urea formaldehyde, melamineformaldehyde, polyamide, glyoxals, stearates and other materialscommercially available for this function.

(p) insolubiliser: generally in levels up to 2% by weight.

For all of the above additives, the percentages by weight quoted arebased on the dry weight of pigment (100%) present in the composition.Where the additive is present in a minimum amount the minimum amount maybe 0.01% by weight based on the dry weight of pigment.

The method according to the fourth aspect of the present invention maybe carried out in a known way which will depend upon the material to becoated, the coating composition to be applied and other factors asdetermined by the operator, such as speed and ease of runnability anduse of a conventional coating machine.

Methods of coating paper and other sheet materials are widely publishedand well known. For example, there is a review of such methods publishedin Pulp and Paper International, May 1994, page 18 et seq. Sheets may becoated on the sheet forming machine. This coating may be “on-machine”,or “off-machine” on a coater or coating machine. Use of high solidscompositions is desirable in the coating method because it leaves lesswater to evaporate subsequently. However, as is well known in the art,the solids level should not be so high that high viscosity and levelingproblems are introduced.

All known methods of coating according a fourth aspect of the presentinvention require (i) a means of applying the coating composition to thematerial to be coated, viz. an applicator; and (ii) a means for ensuringthat a correct level of coating composition is applied, viz. a meteringdevice. When an excess of coating composition is applied to theapplicator, the metering device is downstream of it. Alternatively, thecorrect amount of coating composition may be applied to the applicatorby the metering device such as a film press. At the points of coatingapplication and metering, the paper web support ranges from a backingroll, such as via one or two applicators, to nothing (just tension). Thetime the coating is in contact with the paper before the excess isfinally removed is the dwell time—and this may be short, long orvariable.

The coating is usually added by a coating head at a coating station.According to the quality desired, paper grades are uncoated, singlecoated, double coated and even triple coated. When providing more thanone coat, the initial coat (precoat) may have a cheaper formulation andoptionally less pigment in the coating composition. A coater that isapplying a double coating, such as a coating on each side of the paper,will have two or four coating heads, depending on the number of sidescoated by each head. Most coating heads coat only one side at a time,but some roll coaters (such as a film press, gate roll, size press) coatboth sides in one pass.

Examples of known coaters which may be employed in step (b) include airknife coaters, blade coaters, rod coaters, bar coaters, multi-headcoaters, roll coaters, roll/blade coaters, cast coaters, laboratorycoaters, gravure coaters, kiss coaters, liquid application systems,reverse roll coaters and extrusion coaters.

Embodiments of the present invention will now be described by way ofexample with reference to the following illustrative Examples.

EXAMPLE 1

This example describes the method and crudes used to produce samplepigments for this application. A naturally platy kaolin clay fromGeorgia, USA having a shape factor of about 15 and a particle sizedistribution such that about 60% by weight of the particles had an esdless than 2 microns and about 19% by weight of the particles had an esdless than 0.25 microns was suspended in water. The solids of the kaolinclay in the suspension were 40% solids. The resultant suspension waspassed through a high intensity magnetic separator to removeiron-containing impurities therefrom. After the magnetic separation stepthe kaolin clay slurry was divided into several samples which were thensubjected to relatively gentle attrition grinding in a grinding chamberprovided with a submerged internal impeller, the speed of rotation ofwhich was insufficient to form a vortex in the suspension contained inthe grinding chamber. The grinding medium was a silica sand havinggrains in the size range from 0.6 to 0.85 mm. The grinding was continuedfor a time such that the amount of energy dissipated in the suspensionfor the samples ranged from 20 kWh to 100 kWh per ton of kaolin clay (ona dry weight basis) to produce shape factors for the samples in a rangefrom 35 to 65. After this grinding step, the samples were subjected to aclassification step where the particles were made to have particle sizedistributions such that 85%, 87%, 88% and 92% by weight of the particleshad an esd smaller than 2 μm, and a 25% to 45% by weight of theparticles had an esd smaller than 0.25 μm. The shape factor of thekaolin clay samples were measured individually by the method describedin U.S. Pat. Nos. 5,128,606 and 5,516,617.

The surface area as measured by the BET method was found to range from12.1 m²/g to 19.7 m²/g.

Example 1 illustrates the manner in which kaolin clay crudes can beprocessed to obtain a desired kaolin pigment in accordance with theteachings of the invention.

EXAMPLE 2

This example describes the preparation, application and test results ofcoated paper for rotogravure type of printing. Further samples of a rawkaolin from Georgia, USA were processed similarly to that of Example 1in accordance with the teachings of the invention and the kaolin productwas used as a pigment in a coating composition used for preparing anULWC for rotogravure printing. The physical properties of the crudekaolin appear in Table 1 below.

TABLE 1 Physical Property Value Brightness (G.E.) 80.8 TiO₂ 1.771 Fe₂O₃0.720 % < 2 microns 62.1 % < 0.25 micron 19.6 % < 325 mesh 1.4 ShapeFactor 23.5

The coating composition had the composition shown in Table 2 as follows.

TABLE 2 Ingredient Parts By Weight Pigment under test 100 Syntheticlatex binder 6 Lubricant 1 Dispersant 0.1 Thickener 0.2 Adjusted pH withcaustic 8.5

The synthetic latex binder was a styrene butadiene rubber binder of thetype, which has been found to be suitable for use in rotogravure coatingformulations. The parts by weight shown are parts by weight of latexsolids.

The lubricant was a calcium stearate of the type which is commonly usedin paper coating colors.

The dispersant was a sodium polyacrylate marketed under the trade name“C-211” from Rhone-Poulenc.

The thickener was an alkali swellable acrylic emulsion containinghydrophobic groups and of a type which is found to be suitable for usein rotogravure coating formulations.

Both sides of a base sheet were coated. The coated paper was calenderedas described herein above. Both the pigment product physicalcharacteristics and its effect on the physical properties of the coatedpaper in rotogravure printing are illustrated in Table 3.

TABLE 3 Sample E Sam- Sam- Sam- Sam- (b-fraction Sam- ple A ple B ple Cple D blend) ple F Brightness (G.E.) 85.3 85.7 86.5 86.6 87.7 88.6 % <283.3 89.4 89.6 86.6 89.0 83.0 % <0.25 28.5 32.0 27.9 30.4 34.0 23.0Medium Diameter 0.44 0.38 0.41 0.404 0.44 0.58 Shape Factor 41.7 44.160.8 52.0 58.6 35.9 BET (m2/g) 12.1 16.0 16.3 15.9 16.7 14.9 SheetBrightness, 68.2 67.6 68.7 68.1 67.5 67.9 ISO Opacity, Printers, 88.587.7 88.5 88.3 88.0 87.8 % Sheet Gloss 54.2 52.6 61.0 56.1 56.7 48.8(75°), % PPS Porosity, 18.4 25.2 16.0 20.4 17.3 19.5 ml/s PPS Roughness,1.24 1.40 1.09 1.17 1.14 1.23 1000 kPas % Missing Dot 6.7 10.1 5.4 8.310.9 10.0

Samples A through D are kaolin products of a first embodiment of theinvention, and Sample E is a kaolin product in accordance with a secondembodiment of the invention comprising a blend of 80% by weight of anaturally platy clay and 20% by weight b-fraction kaolin clay-component.The b-fraction component was blended into the naturally platy crude clayafter the blunging process. These b-fractions represent the coarseunderflow from a centrifuge during the standard production of #1 coatingclays.

Sample F is a delaminated product of the prior art. Sample F is adelaminated kaolin clay from Georgia, USA having a particle sizedistribution such that 81.5% by weight consisted of particles having anesd smaller than 2 μm, 62.5% by weight consisted of particles having anesd smaller than 1.0 μm and 20% consisted of particles having an esdsmaller than 0.25 μm. The particle shape factor of the product was 35.9.

The kaolin products, Samples A through E, made in accordance with theembodiments of the invention were compared with the commerciallyavailable delaminated product, Sample F. These pigments were eachseparately made into the composition shown in Table 2 above.

Each composition was coated onto base paper of substance weight 35 g·m⁻²by means of a coating machine of the type described in GB-A-1032536fitted with a short dwell time head. The paper speed was 800 m·min⁻¹.Samples of a coated paper were prepared at different coat weights in therange of from about 3 g·m⁻² to about 10 g·m⁻² The coated paper was driedand then subjected to calendering by passage three times between therolls of a supercalender at a temperature of 150° C. and a pressure of200 psi.

The samples of calendered coated paper prepared from each of the coatingcompositions for the several samples A through F were then tested forsheet gloss; sheet brightness; opacity; Parker Print Surf (PPS)porosity; Parker Print Surf (PPS) roughness using the soft backingmaterial and a pressure of 1000 kPA; and the missing dot test (known tothose skilled in the art). The results are set forth in Table 3 above.

From Table 3 is can be seen that the pigments of the invention provideULWC papers suitable for use in rotogravure printing, which papersgenerally have improved gloss, brightness, porosity, opacity, andsmoothness as compared with a coated paper which has been prepared usingthe commercially available delaminated pigment which is generallyrecommended for preparing coated papers of this type. It will be noted,in particular, that commercially available pigment (Sample F) has aparticle size distribution which closely resembles that of the pigmentsof the invention, and it would generally be expected that this pigmentof the prior art would have approximately equal performance to that ofthe pigments in accordance with the invention when used as a pigment ina paper coating composition. However the pigments of the invention, insome instances, are seen to be superior even to this commerciallyavailable delaminated pigment. A comparison of performance of Samples B,C and D, which have similar 2 μm contents, but differ significantly inthe shape factor, shows that the higher shape factor pigment (C) has 7.5units higher sheet gloss, about half the missing dots, 1 unit bettersheet brightness and opacity than the lowest shape factor pigment (B).The performance of pigment D is intermediate. All the higher shapefactor pigments have significantly improved performance over thecommercially available delaminated product (F). A comparison of SamplesA and B shows that an increase in the 2 micron content does not resultin an improvement in sheet properties, if the shape factor remains thesame. This result directly demonstrates the need to apply the teachingsof the invention to produce a pigment product with the requiredattributes to obtain an improvement in sheet properties. It takes anincrease in shape factor as demonstrated by Samples C, D, and E (theinvention) to lead to significantly improved performance.

EXAMPLE 3

This example describes the composition of coating, application and testresults of coated paper for offset type of printing application. Samplessimilar to Example 3, that is Samples A through E (products of theinvention) and Sample F (commercially delaminated product) of Example 2were used in a coating composition used for preparing an ULWC for offsetprinting. The coating composition had the composition shown in Table 4as follows:

TABLE 4 Ingredient Parts By Weight Pigment under test 100 Starch 8Synthetic latex binder 8 Lubricant 1 Dispersant 0.1 Thickener 0.1Adjusted pH with caustic 8.5

The pigments, lubricant, dispersant, and thickener were the same asthose used in Example 3. The starch was a preconverted ethylated productof a type which is found to be suitable for use in offset coatingformulations.

The synthetic latex binder was a styrene butadiene rubber binder of thetype which has been found to be suitable for use in offset coatingformulations. The parts by weight shown are parts by weight of latexsolids.

The kaolin products, Samples A through F of the invention are comparedwith the commercially delaminated product (Sample G), and each wereseparately made into the composition shown in Table 4 above and thenapplied to a base sheet in offset printing in a manner similar to thatdescribed in Example 2.

The samples of calendered coated paper prepared from each of the eightcoating compositions were then tested for sheet brightness, opacity,sheet gloss, PPS porosity and smoothness, and print gloss using themethods described hereinabove.

The physical properties of the pigment product used in the coatingcompositions and the properties of the paper coated with these pigmentproducts are shown in Table 5.

TABLE 5 Sample F (b-Fraction Sample A Sample B Sample C Sample D SampleE Blend) Sample G Brightness 85.3 85.7 86.5 86.6 86 87.7 88.6 (G.E.) %<2 83.3 89.4 89.6 88.6 85 89.0 83.0 % <0.25 28.5 32.0 27.9 30.4 31 34.023.0 Median Particle 0.44 0.38 0.411 0.404 0.415 0.44 0.58 Size ShapeFactor 41.7 44.1 60.8 52.0 51 58.6 35.9 BET (m2/g) 12.1 16.0 16.3 15.916 16.7 14.9 Sheet 66.0 66.2 66.4 66.0 66 65.6 65.3 Brightness, ISOOpacity, 87.6 87.7 87.9 87.4 87 87.2 57.1 Printers, % Sheet Gloss 45.649.0 54.7 49.7 47 51.9 43.2 (75 0), % PPS Porosity 5.0 5.3 4.5 4.9 5 4.55.4 (ml/min) PPS 1.25 1.26 1.14 1.20 1.25 1.18 1.26 Roughness, 1000 kPasPrint Gloss 64.2 68.1 70.8 66.4 64 71.6 62.9

It can be seen from Table 5 that the pigments of the invention provideULWC papers suitable for use in offset printing. Compared to thecommercial delaminated clay all the pigments have significantly improvedsheet brightness, opacity, sheet gloss, print gloss and smoothness.Samples B, C and D have similar 2 μm content with significantlydifferent shape factors. It is clearly seen that the highest shapefactor pigment(C) has up to 5 units higher sheet gloss compared to thesample with the lower shape factor (B). Samples B, C and D havesignificantly higher sheet gloss compared to Sample G, which is thecommercial delaminated clay sample. The samples with better sheet glossalso have better print gloss. Sample A which has similar 2 μm content,higher 0.25 μm content, and higher shape factor as compared to thecommercial delaminated clay (Sample G), has 2.5 units higher sheet glossand print gloss. This comparison demonstrates that clay producedaccording to the teachings of the invention still outperformcommercially available delaminated clay. Sample F made from the secondpreferred pigment, also has improved behavior similar to Sample C. Acomparison of Samples A and E which have similar 2 μm and 0.25 μmcontents but a different shape factor, shows that the higher shapefactor pigment of Sample E has improved performance.

What is claimed is:
 1. A pigment product for a paper coatingcomposition, said pigment product comprising a particulate kaolinprocessed from a naturally platy clay having a shape factor of at least15, said pigment product having particles with a particle sizedistribution such that at least 85% by weight of the particles have anequivalent spherical diameter less than 2 μm and not more than 35% byweight of the particles have an esd less than 0.25 μm and whoseparticles have a shape factor of at least 50, and wherein said pigmentproduct further comprises a b-fraction component having a shape factorless than 15 and having particles with a particle size distribution suchthat less than 50% by weight have an esd less than 2 μm and less than20% by weight have an esd less than 0.25 μm.
 2. A pigment productaccording to claim 1 wherein said processed particulate kaolin isprocessed from a crude comprising a blend including both a naturallyplaty clay crude with a shape factor greater than 15 and said b-fractionhaving a shape factor less than
 10. 3. A pigment product according toclaim 2 wherein said naturally platy clay crude has a particle sizedistribution such that greater than 50% by weight of the particles havean esd less than 2 μm and less than 25% by weight of the particles navean esd less than 0.25 μm.
 4. A pigment product according to claim 2wherein said blend has a particle size distribution such that greaterthan 50% by weight of the particles have an esd less than 2 μm and lessthan 25% by weight of the particles have an esd less than 0.25 μm.
 5. Apigment product for a paper coating composition, said pigment productcomprising a particulate kaolin processed from a naturally platy clayhaving a shape factor of at least 15, said pigment product havingparticles with a particle size distribution such that at least 85% byweight of the particles have an equivalent spherical diameter less than2 μm and not more than 35% by weight of the particles have an esd lessthan 0.25 μm and whose particles have a shape factor of at least
 60. 6.A pigment product for a paper coating composition, said pigment productcomprising a particulate kaolin processed from a naturally platy clayhaving a shape factor of at least 15, said pigment product havingparticles with a particle size distribution such that between about 87%to about 92% by weight of the particles have an esd less than 2 μm, andbetween about 20% to about 35% by weight of the particles have an esdless than 0.25 μm and whose particles have a shape factor of at least50.
 7. A pigment product for a paper coating composition, said pigmentproduct comprising a particulate kaolin processed from a naturally platyclay having a shape factor of at least 15, said pigment product havingparticles with a particle size distribution such that at least 85% byweight of the particles have an equivalent spherical diameter less than2 μm and not more than 35% by weight of the particles have an esd lessthan 0.25 μm and whose particles have a shape factor of at least
 55. 8.A method of producing a pigment product suitable for use in a coatingcomposition to provide a gloss coating on paper, the method comprising:mixing a raw kaolin clay crude, comprising a blend of a naturally platyclay having a shape factor of at least 15 and a b-fraction claycomponent having a shape factor less than 10 to produce a particle sizedistribution such that greater than 50% by weight of the particles havean esd less than 2 μm and not more than 25% by weight of the particleshave an esd less than 0.25 μm, with water to form an aqueous suspensioncontaining from about 20% to about 70% of kaolin on a dry weight basis;diluting the aqueous suspension to 40% of kaolin on a dry weight basisthen subjecting the suspension produced to attrition grinding using aparticulate grinding medium for a time sufficient to dissipate in thesuspension an optimum amount of energy to impart an average shape factorto the kaolin clay particles of at least 50; separating the aqueoussuspension of ground kaolin clay from the particulate grinding medium;classifying particles in the aqueous suspension by size; and dewateringthe aqueous suspension to recover said pigment product thereof.
 9. Amethod according to claim 8 wherein the crude is initially crushed andthen mechanically worked by blunging in water.
 10. A method according toclaim 9, wherein the water contains an inorganic or organic claydispersant.
 11. A method according to claim 10 wherein said claydispersant is a mixture of sodium hexametaphosphate and soda ash.
 12. Amethod according to claim 9, wherein the solids of the blunged clay arefrom 20% to 65% by weight of kaolin on a dry weight basis.
 13. A methodaccording to claim 9, wherein the solids of the blunged clay are from40% to 60% by weight of kaolin on a dry weight basis.
 14. A methodaccording to claim 8 wherein prior to or after the attrition grinding,one or more beneficiation processes are performed on the clay suspensionor slurry.
 15. A method according to claim 14 wherein said beneficiationprocesses are chosen from magnetic separation, ozone, reduced-acidleaching, flotation, selective flocculation, and combinations thereof.16. A method according to claim 8, further comprising degritting theclay slurry prior to diluting the aqueous suspension to remove coarseparticles greater than 45 microns and subjecting the clay slurry to ahigh intensity magnetic separator to remove the iron-bearing mineralsfrom the clay.
 17. A method according to claim 8, wherein the shapefactor of the naturally platy clay is greater than
 25. 18. A methodaccording to claim 8, wherein the shape factor of the naturally platyclay is greater than
 30. 19. A method according to claim 8, wherein theenergy dissipated during grinding is between about 20 kWh to about 100kWh of energy per ton of kaolin present on a dry weight basis.
 20. Amethod according to claim 8, wherein the energy dissipated duringgrinding is between about 25 kWh to about 50 kWh of energy per ton ofkaolin present on a dry weight basis.
 21. A method according to claim 8,wherein the specific gravity of the particulate grinding medium is 2 ormore.
 22. A method according to claim 8, wherein the particulategrinding medium comprises grains of silica sand having diameters notlarger than about 2 mm and not smaller than about 0.25 mm.
 23. A methodaccording to claim 8, wherein, after said particle size classification,the kaolin clay particles have a particle size distribution such that atleast 80% of the particles have an esd less than 2 μm.
 24. A methodaccording to claim 8 wherein, after said particle size classification,the kaolin clay particles have a particle size distribution such thatfrom 85% to 95% of the particles have an esd less than 2 μm.
 25. Amethod according to claim 8, wherein, after said particle sizeclassification, the kaolin clay particles have a particle sizedistribution such that from 88% to 92% of the particles have an esd lessthan 2 μm.
 26. A method according to claim 8, wherein the resultingpigments have a particle size distribution of at least 58% by weightfiner than 0.5 μm.
 27. A method according to claim 8, wherein theresulting pigments have a particle size distribution of from 50% to 60%by weight finer than 0.5 μm.
 28. A method according to claim 8, whereinthe resulting pigments have a particle size distribution of from 53% to58% by weight finer than 0.5 μm.
 29. A method according to claim 8,wherein the resulting pigments have a particle size distribution of atleast 38% by weight finer than 0.3 μm.
 30. A method according to claim8, wherein, after the particle size classification, the kaolin clayparticles have a particle size distribution such that less than 35% ofthe particles have an esd less than 0.25 μm.
 31. A method according toclaim 8, wherein after said particle size classification, the kaolinclay particles have a particle size distribution such that from about20% to 35% of the particles have an esd less than 0.25 μm.
 32. A methodaccording to claim 8, wherein after said particle size classification,the kaolin clay particles have a particle size distribution such thatfrom about 25% to 35% of the particles have an esd less than 0.25 μm.33. A method according to claim 8, wherein the resulting pigments havean average particle size from about 0.30 to about 0.60 μm.
 34. A methodaccording to claim 8, wherein the resulting pigments have an averageparticle size from about 0.35 to about 0.52 μm.
 35. A method accordingto claim 8, wherein the resulting pigments have an average particle sizefrom about 0.38 to about 0.45 μm.
 36. The method according to claim 8,further comprising, prior to said dewatering, subjecting the suspensionto a leaching process to remove iron oxides and other leachable colorantspecies to change the shade of the clay.
 37. The method according toclaim 8, wherein after dewatering, the solids content of the slurry israised to about 60% by weight.
 38. A coating composition for use inproducing gloss coatings on paper and other substrates, whichcomposition comprises an aqueous suspension of a particulate pigmenttogether with a hydrophilic adhesive, wherein the particulate pigmentcomprises a pigment product according to claim
 2. 39. A method ofproducing a gloss coating on paper, comprising applying the coatingcomposition of claim 8, to coat a sheet of paper and calendaring thepaper to form a gloss coating thereon.
 40. A pigment product for a papercoating composition, said pigment product comprising a particulatekaolin processed from a naturally platy clay having a shape factor of atleast 15, said pigment product having particles with a particle sizedistribution such that at least 85% by weight of the particles have anequivalent spherical diameter less than 2 μm and not more than 35% byweight of the particles have an esd less than 0.25 μm and whoseparticles have a shape factor of at least 50 and a surface area greaterthan 12 m²/g when measured by the BET method.
 41. A method of making apigment product for a paper coating composition comprising: providing asource of naturally platy clay, which is from a sedimentary deposit andhas particle size distribution such that greater than 50% by weight ofthe particles have an esd less than 2 μm and not more than 25% by weightof the particles have an esd less than 0.25 μm; forming a kaolin pigmenthaving particles with a particle size distribution such that at least85% by weight of the particles have an equivalent spherical diameterless than 2 μm and not more than 35% by weight of the particles have anesd less than 0.25 μm and whose particles have a shape factor of atleast 50; wherein said forming comprises: initially crushing the crudethen mechanically working the crude by blunging in water; mixing saidnaturally platy clay with water to form an aqueous suspension containingfrom 20% to about 70% of kaolin on a dry weight basis; diluting theaqueous suspension to about 40% of kaolin on a dry weight basis and thensubjecting the aqueous suspension to attrition grinding using aparticulate grinding medium for a time sufficient to dissipate in theaqueous suspension an optimum amount of energy to impart an averageshape factor to the kaolin clay particles of at least 50; separating theaqueous suspension of ground kaolin clay from the particulate grindingmedium; classifying particles in the aqueous suspension by size; anddewatering the aqueous suspension to recover said pigment productthereof.
 42. A method of making a pigment product for a paper coatingcomposition comprising: providing a source of naturally platy clay,which is from a sedimentary deposit and has particle size distributionsuch that greater than 50% by weight of the particles have an esd lessthan 2 μm and not more than 25% by weight of the particles have an esdless than 0.25 μm; forming a kaolin pigment having particles with aparticle size distribution such that at least 85% by weight of theparticles have an equivalent spherical diameter less than 2 μm and notmore than 35% by weight of the particles have an esd less than 0.25 μmand whose particles have a shape factor of at least 50; wherein saidforming comprises: mixing said naturally platy clay with water to forman aqueous suspension containing from 20% to about 70% of kaolin on adry weight basis; degritting the aqueous suspension to remove coarseparticles greater than 45 μm and subjecting the clay slurry to a highintensity magnetic separator to remove the iron-bearing minerals fromthe clay; diluting the aqueous suspension to about 40% of kaolin on adry weight basis and then subjecting the aqueous suspension to attritiongrinding using a particulate grinding medium for a time sufficient todissipate in the aqueous suspension an optimum amount of energy toimpart an average shape factor to the kaolin clay particles of at least50; separating the aqueous suspension of ground kaolin clay from theparticulate grinding medium; classifying particles in the aqueoussuspension by size; and dewatering the aqueous suspension to recoversaid pigment product thereof.
 43. A method of making a pigment productfor a paper coating composition comprising: providing a source ofnaturally platy clay, which is from a sedimentary deposit and hasparticle size distribution such that greater than 50% by weight of theparticles have an esd less than 2 μm and not more than 25% by weight ofthe particles have an esd less than 0.25 μm; forming a kaolin pigmenthaving particles with a particle size distribution such that at least85% by weight of the particles have an equivalent spherical diameterless than 2 μm and not more than 35% by weight of the particles have anesd less than 0.25 μm and whose particles have a shape factor of atleast 50; wherein said forming comprises: mixing said naturally platyclay with water to form an aqueous suspension containing from 20% toabout 70% of kaolin on a dry weight basis; diluting the aqueoussuspension to about 40% of kaolin on a dry weight basis and thensubjecting the aqueous suspension to attrition grinding using aparticulate grinding medium, wherein the specific gravity of theparticulate grinding medium is 2 or more, for a time sufficient todissipate in the aqueous suspension an optimum amount of energy toimpart an average shape factor to the kaolin clay particles of at least50; separating the aqueous suspension of ground kaolin clay from theparticulate grinding medium; classifying particles in the aqueoussuspension by size; and dewatering the aqueous suspension to recoversaid pigment product thereof.
 44. A method according to claim 43,wherein the particulate grinding medium comprises grains of silica sandhaving diameters not larger than about 2 mm and not smaller than about0.25 mm.
 45. A method of making a pigment product for a paper coatingcomposition comprising: providing a source of naturally platy clay,which is from a sedimentary deposit and has particle size distributionsuch that greater than 50% by weight of the particles have an esd lessthan 2 μm and not more than 25% by weight of the particles have an esdless than 0.25 μm; forming a kaolin pigment having particles with aparticle size distribution such that at least 85% by weight of theparticles have an equivalent spherical diameter less than 2 μm and notmore than 35% by weight of the particles have an esd less than 0.25 μmand whose particles have a shape factor of at least 50 and an averageparticle size from about 0.30 to about 0.60 μm; wherein said formingcomprises: mixing said naturally platy clay with water to form anaqueous suspension containing from 20% to about 70% of kaolin on a dryweight basis; diluting the aqueous suspension to about 40% of kaolin ona dry weight basis and then subjecting the aqueous suspension toattrition grinding using a particulate grinding medium for a timesufficient to dissipate in the aqueous suspension an optimum amount ofenergy to impart an average shape factor to the kaolin clay particles ofat least 50; separating the aqueous suspension of ground kaolin clayfrom the particulate grinding medium; classifying particles in theaqueous suspension by size; and dewatering the aqueous suspension torecover said pigment product thereof.
 46. A method according to claim45, wherein the resulting pigments have an average particle size fromabout 0.35 to about 0.52 μm.
 47. A method according to claim 45, whereinthe resulting pigments have an average particle size from about 0.38 toabout 0.45 μm.
 48. A method of making a pigment product for a papercoating composition comprising: providing a source of naturally platyclay, which is from a sedimentary deposit and has particle sizedistribution such that greater than 50% by weight of the particles havean esd less than 2 μm and not more than 25% by weight of the particleshave an esd less than 0.25 μm; forming a kaolin pigment having particleswith a particle size distribution such that at least 85% by weight ofthe particles have an equivalent spherical diameter less than 2 μm andnot more than 35% by weight of the particles have an esd less than 0.25μm and whose particles have a shape factor of at least 50; wherein saidforming comprises: mixing said naturally platy clay with water to forman aqueous suspension containing from 20% to about 70% of kaolin on adry weight basis; diluting the aqueous suspension to about 40% of kaolinon a dry weight basis and then subjecting the aqueous suspension toattrition grinding using a particulate grinding medium for a timesufficient to dissipate in the aqueous suspension an optimum amount ofenergy to impart an average shape factor to the kaolin clay particles ofat least 50; separating the aqueous suspension of ground kaolin clayfrom the particulate grinding medium; classifying particles in theaqueous suspension by size; dewatering the aqueous suspension to recoversaid pigment product thereof; and raising the solids content of theaqueous suspension to about 60% by weight.